Seawater intrusion and methane sequestration followed the retreat of the Fennoscandian ice sheet

The efficiency of submarine groundwater discharge (SGD) in transporting solutes into coastal environments during glacial periods remains poorly understood. Moreover, the absence of observational constraints on offshore groundwater emplacement times hinders our understanding of glacial-driven SGD timescales and subsequent solute fluxes. This knowledge gap presents challenges in predicting the impact of ice sheet collapse on critical solute discharge into peripheral oceans. An SGD site with methane seepage offshore northern Norway that experienced drastic changes due to Fennoscandian ice sheet dynamics offers insights into glacial-interglacial transitions and their consequences for offshore groundwater circulation. Radiocarbon (¹⁴C) contents of the dissolved inorganic carbon along with chlorinity contents of the upward-advected fluids reveal that the groundwater transit times of the seawater component coincide with the retreat of the Fennoscandian ice sheet from the continental shelf. This suggests that seawater intrusion replaced offshore freshening, flushing the freshened aquifer with seawater. Decelerating groundwater discharge velocities and aquifer salinization as a consequence of glacial unloading allowed the precipitation of authigenic carbonates, sequestering discharged methane. Reduced groundwater advection velocities facilitated the migration of the sulfate-methane transition zone into the marine sediments, while the aquifer salinization likely increased Ca²⁺ concentrations, promoting carbonate precipitation. Our geochemical evidence conclusively shows that the decreased hydraulic head gradients, coupled with aquifer salinization, mitigated the escape of methane from the subsurface.